I have had my blog, www.sourhumor.com, for about 2 months now. I started from scratch and I have been posting everyday, submitting to directories, SEO, etc. Anyway, this month I have been getting 60-120 uniques a day. I just started using StumbleUpon and reviewing my posts everday, now I am getting double, almost triple that. Also, that traffic earned me money with AdBrite which hasnt happened before. So all in all, StumbleUpon.com is a great tool for bloggers to get the traffic they deserve.
Yes once posts get stumbled they tend to get re-stumbled again and again, my best source of traffic. Sometimes you get a great peak too - like this one: Busiest day: 5,330 — Tuesday, March 18, 2008